Ca influences nitrogen assimilation and protein metabolism
Plant growth requires a source of nitrogen for the biosynthesis of a variety of nitrogen-containing biomolecules, such as amino acids and nucleic acids. Nitrogen deficiency leads to crops productivity and quality reduction.43Previous study has reported Ca mediated nitrogen assimilation in plant.44 Our study inP. massoniana revealed that exogenous Ca may activate the expression of glutamine synthetase (GS; spots 9 and 10), and motivate GS mediated nitrogen assimilation pathway.45
Apart from nitrogen assimilation related protein, a number of protein metabolism related proteins were detected with changed expression patterns at different Ca levels (Table 1), which involved in protein synthesis, folding, destination and degradation. The up-regulated mRNA translation and peptide elongation related proteins asparagine-tRNA ligase (spot 49) and glycine-tRNA ligase 1 (spot 50) under high Ca condition suggests Ca perhaps benefit the binding of amino acid and tRNA and contribute to protein biosynthesis. The increased soluble protein content at high Ca level (Figure 1F) is an evidence for this speculation.
Four heat shock protein (HSP) 70 related proteins were identified (spot54-56, 64), whose abundances showed markedly increment at high Ca level (Table 1). HSP 70 is a diverse function protein. Apart from stress response, it also involves in plenty of other biological processes, such as protein folding, assorting and translocation, and the development and differentiation of various plant tissues.46Consistently, HSP70 owns the highest degree in the PPI network (Figure 5), showing its versatile roles in diverse biological processes. Adequate Ca treatment stimulated HSP 70 high expression possibly facilitates the aforementioned biological processes in P. massoniana seedlings, especially protein folding and transportation related process.
Peptidyl-prolyl cis-trans isomerase (PPIase) catalyzes the reversible conversion of the peptidyl-prolyl bond from cis to trans, which is a rate-limiting step in the folding of proteins.47 Previous study in Arabidopsis reported overexpression of PPIase gene confers stress tolerance to heat, ABA, drought and salt.48 Our study inP. massoniana showed the expression of PPIase CYP20-2 increased ten folds under Ca deficiency condition (Table 1). Up-regulated PPIase may be a potential approach for P. massoniana to cope with Ca deficiency, whose underlying mechanism is associated to PPIase in correct protein folding. It is noticeable that high Ca treatment induced drastically soaring of PPIase CYP20-2 as well. This phenomenon can be attributed to high Ca improved protein synthesis and seedling growth, more PPIase is necessary to ensure the proper folding and packaging of accelerating synthesized proteins.